This proposal concerns studies of the retina as a model system of a neural tissue and as a system for the study of biochemical control mechanisms in this tissue. Glutamine Synthetase (GS) and other enzymes, and their possible relation to the visual function of the retina, as well as general retinal metabolism, are a major focus. GS, which is present in high concentrations in neural retina, can be prematurely induced in embryonic chick retina by glucocorticoids; this induction is inhibited by L-glutamate. L-glutamate severely damages embryonic and neonatal retina both in vivo and in culture. The primary emphasis of the current and future work is to attempt to elucidate the biochemical and morphological effects of monosodium glutamate on retinal tissue. One approach is to examine different glutamate analogues to determine the structural requirements for the morphological damage and for effects on GS induction, as well as to clarify whether these effects are related. Attempts to find the physiological substrate of the gamma-glutamyltransferase (GT) activity of GS have been continued. The enzyme has been localized in glial cells in rat embryo hypothalamic cells in cultures; experiments attempting to localize GS/GT activity in the developing chick retina will be continued. GS/GT inducibility and steroid receptors, in whole compared to dissociated retinal tssue, will be further investigated. Experiments comparing control of GS/GT activity in normal retina and in a malignant retinal cell line are also planned. Future experiments will also include (1) studies of GS/GT activity in ocular tissue other than retina, (2) comparison of enzyme activity in various species, (3) study of possible enzyme alterations in normal and diseased retin, and (4) a study of effects of cortisol and certain sugars on surface properties of retinal cells in culture.